In recent years, mobile electronic devices have rapidly been spreading, and along with the progress in performance of these devices, lithium ion secondary batteries used therein have been required to have higher output and higher energy density. Moreover, the development into environmental protection energy and alternative energy to oil for electric vehicles (EV) has also been expected recently, and for example, an alkali secondary battery (Ni—MH battery) and a nonaqueous electrolyte secondary battery including a lithium compound (lithium ion battery) have been put into practical use.
A positive electrode and a negative electrode of a secondary battery such as a Ni—MH battery or a lithium ion battery are fabricated by binding each active material for the positive electrode and the negative electrode to a current collector with a binder material. In the example of the lithium ion battery, at first, a solution in which polyvinylidene fluoride (PVdF) is dissolved in N-methyl-2-pyrrolidone (NMP) or an aqueous dispersion liquid of polytetrafluoroethylene (PTFE) was used as the binder material for each of the positive electrode and the negative electrode. However, since PVdF has a low binding force, it was necessary to use a large amount of PVdF for the active material and the capacity per unit volume was low. Moreover, PVdF is easily decomposed by dehydrofluorination reduction in a reducing atmosphere, and the generated hydrogen fluoride might cause the deterioration of the active material.
Therefore, the use of an aqueous dispersion base styrene butadiene latex binder material (aqueous dispersion based SBR) with a stronger binding property has recently become the mainstream. However, this aqueous dispersion based SBR cannot be used in the positive electrode where oxidation reaction occurs, because a double bond of butadiene causes oxidative degradation.
Therefore, as a binder material that can be also used for the positive electrode, an olefin based polymer binder that is electrochemically stable and has a low swelling property with respect to an electrolytic solution has been proposed.